700 THE PHYSIOLOGY OF REPRODUCTION 



to a stimulus which may be either internal or external. The observa 

 tions which Riddle, Steinach, and others have made upon animals of 

 many different kinds point even to the possibility that sex may be 

 reversed after it has once been established. 



It seems certain that sex is not determined by the same factors 

 in all cases, neither is it determined at the same period of development. 

 It may well be that some gametes have an initial tendency to give 

 rise to males and others to give rise to females, and to this extent it 

 is probably legitimate to speak of male and female ova or male and 

 female spermatozoa. Moreover, the conclusion is probably correct 

 that these are developed (at least generally) in simple Mendelian 

 ratios. But it is also probable that no gamete is either purely male 

 or purely female, and it is possible that in some the two kinds of 

 sexual determinants or tendencies are about equally represented. 



When once we admit the existence of latent (i.e. recessive) sexual 

 characters in individuals in which the characters of one sex are 

 dominant, and that under certain circumstances those of the latent 

 sex can develop at the expense of the dominant ones, in response to 

 appropriate physiological stimuli, we are compelled to acknowledge 

 also that the sex of the future individual is not always predetermined 

 HI the gametes or even in the fertilised ovum, but may be called into 

 being at a later stage of life. 



Such an admission is of course opposed to some extent to the 

 modern tendency to believe that sex is fixed irrevocably in the 

 fertilised ovum or in the gametes before fertilisation. But while there 

 is evidence amounting to proof that this is the case in some forms of 

 life, it does not necessarily follow that it is true of all Metazoon 

 animals, or even that it is uniformly so of the particular species 

 which have been investigated. On the other hand, many of the facts 

 enumerated above point to the conclusion that the sex of the future 

 organism is determined in different cases by different factors and at 

 different stages of development either in the unfertilised gamete, or 

 at the moment of fertilisation, or in the early embryo. 



Finally, there is evidence that a change in the metabolism, even 

 in comparatively late life, whether induced by castration or the 

 introduction of another gonad or arising primarily from a different 

 cause, may initiate changes in the direction of the opposite sex, or 

 even bring about a complete sex- reversal. 1 



1 For a complete review of Champy's work on Triton, see Champy, " Etude 

 experimentale sur les Differences sexuelles chez les Tritons," Arch. d. Morph. 

 Exper. (Fasc vii.), 1922. See also Swingle, " Is there Transformation of Sex 

 in Frogs?" Amer. Jfat., vol. Ivi., 1922. The latter author throws doubt on 

 Witschi's conclusions. 



